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Author Topic:   Worm Gear fault frequency calulation
ACrooks
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posted 01-08-2004 05:29 PM     Click Here to See the Profile for ACrooks   Click Here to Email ACrooks     Edit/Delete Message   Reply w/Quote
How do I calculate the gear mesh and other fault frequencies for a worm gear elevator hoist motor? The name plate states 79:2 and the ring has 79 teeth so the 2 must refer to the worm somehow. The input is 1736 rpm, but I didn't have my photo tach available at the time to get output speed. Let me know if more info is needed.

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Jon Chandler
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posted 01-08-2004 05:51 PM     Click Here to See the Profile for Jon Chandler   Click Here to Email Jon Chandler     Edit/Delete Message   Reply w/Quote
The worm can have from 1 to several "starts" or threads in parallel. A screw has a single thread around it. A thread with 2 starts has 2 threads, spaced at 180 degrees from each other and so on.

Each turn of a worm with one start advances one tooth on the wheel; if there are 2 starts, 2 teeth advance per turn, and so on.

Gear mesh frequency is therefore equal to the worm speed x number of starts. This is usually 1x rotation, but in this case will be twice rotation. Not an easy task to separate gear mesh from balance and alignment problems on a worm gear.

As was discussed in a recent thread, you usually don't see much in the way of gear mesh on this type of gear set anyway. So many gear teeth are in contact with the worm at any time that the energy transfer is pretty smooth and no vibration results.

The best diagnostic tool for worm gears is probably oil analysis.

Jon
Spintelligent Labs

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ACrooks
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posted 01-09-2004 10:49 AM     Click Here to See the Profile for ACrooks   Click Here to Email ACrooks     Edit/Delete Message   Reply w/Quote
Jon Thanks for the reply. The worm only has one start on it that I could see. I believe the nameplate said that so you would know it had 79 teeth on the ring or so it didn't say 39.5. The problem we are having is a terrible noise that sounds like a gearbox that's chunking teeth off. We don't have any bearing type info for the free end rolling element thrust bearings but they appeared to be double angular contact ball bearings and a sleeve bearing on the drive end. The vibration spectrum is dominated by 7.42 orders harmonics. I just can't see this being gear related. I think the elevator mechanic screwed up the preload on the thrust bearing and that's why this energy is present. Oh info I forgot to include this. The hoist gearbox did seize up two weeks ago. The drive end sleeve bearing is actually what seized up. They disassembled it and brought it to our shop where we replace the sleeve bearing and rebuilt the journal. We then balanced the armature and sent it on its way. The elev tech replaced the thrust bearings and then reassembled it. That is when the normal maintenance guy said this noise started. He also said the gearbox sounded fine about a week before if crapped the bed. The brass ring looks in good condition with no signs of grinding. The steel worm looks good with nice polished looking teeth. The only thing I can see this 7.42 orders energy being generated by is an improperly installed thrust bearing assembly. Here are some plots. http://reliability-magazine.com/pub/C:\My Documents\275 Elev 1 worm gear free end axial.doc

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ACrooks
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posted 01-09-2004 12:40 PM     Click Here to See the Profile for ACrooks   Click Here to Email ACrooks     Edit/Delete Message   Reply w/Quote
Try this http://reliability-Magazine.com/pub/C:\275Elev1.doc

Hmm I wonder why this won't work.

[This message has been edited by ACrooks (edited 01-09-2004).]

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Jon Chandler
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posted 01-09-2004 12:53 PM     Click Here to See the Profile for Jon Chandler   Click Here to Email Jon Chandler     Edit/Delete Message   Reply w/Quote
Here's the correct link:

Try this http://reliability-Magazine.com/pub/275Elev1.doc

It can be a pain in the neck getting these to work.

I think you're right on the money with the improperly adjusted pre-load. I'll bet things are banging around loose in the gearbox.

Inspecting the worm and wheel for excessive play will probably reveal the problem. Oil analysis after the unit is back together and operating for a short period might be worth the cost to be sure the wheel isn't wearing too quickly.

Jon
Spintelligent Labs

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Good Vibrations
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posted 01-09-2004 02:35 PM     Click Here to See the Profile for Good Vibrations   Click Here to Email Good Vibrations     Edit/Delete Message   Reply w/Quote
I have used the following formula to find Gear Mesh for Worm Gear boxes.

In: would be the worm.
Out: would be the wheel.
This example has 24 teeth on the wheel and an output speed of 10Hz.

(#T) out * (T.S.) out = GMF
(24) * (10Hz.) = 240 Hz

Then the number of flights (#F) on the input gear can be determined as follows.

GMF / (TS) in = (#F)in
240Hz. / 29.5Hz. = 8.13

This worm gear has 8.13 flights meshing with the 24 teeth on the output gear during its 1 revolution.

Mike

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